This invention relates to seismic exploration and processing, and more specifically to determining the seismic data quality for a plurality of locations in a given seismic survey.
In the petroleum industry, seismic prospecting techniques are commonly used to aid in the search for and the evaluation of subterranean hydrocarbon deposits. In seismic prospecting, one or more sources of seismic energy emit waves into a subsurface region of interest, such as a geologic formation. These waves enter the formation and may be scattered, e.g., by reflection or refraction, by subsurface seismic reflectors (i.e., interfaces between underground formations having different elastic properties). The reflected signals are sampled or measured by one or more receivers, and the resultant data is recorded. The recorded samples may be referred to as seismic data or a “seismic trace”. The seismic data may be analyzed to extract details of the structure and properties of the subsurface region of the earth being explored.
Seismic prospecting consists of three separate stages: data acquisition, data processing and data interpretation. The success of a seismic prospecting operation depends on satisfactory completion of all three stages.
In general, the purpose of seismic exploration is to map or image a portion of the subsurface of the earth (a formation) by transmitting energy down into the ground and recording the “reflections” or “echoes” that return from the rock layers below. The energy transmitted into the formation is typically sound energy. The downward-propagating sound energy may originate from various sources, such as explosions or seismic vibrations on land, or air guns in marine environments. Seismic exploration typically uses one or more energy sources and typically a large number of sensors or detectors. The sensors that may be used to detect the returning seismic energy are usually geophones (land surveys) or hydrophones (marine surveys).
One example of a seismic survey that is used in the art is three-dimensional (“3D”) seismic exploration. In 3D seismic exploration survey lines and seismic arrays are closely spaced to provide detailed subsurface coverage. With this high density coverage, extremely large volumes of digital data need to be recorded, stored and processed before final interpretations can be made. Processing requires extensive computer resources and complex software to enhance the signal received from the subsurface and to mute accompanying noise which masks the signal.
After the data is processed, scientists and engineers assemble and interpret the 3D seismic information in the form of a 3D data cube which represents a display of subsurface features. Using this data cube, information can be displayed in various forms. Horizontal time slice maps can be made at selected depths. Using a computer workstation, an interpreter can also slice through the data cube to investigate reservoir issues at different seismic horizons. Vertical slices or cross-sections can also be made in any direction using seismic or well data. Seismic picks of reflectors can be contoured, thereby generating a time horizon map. Time horizon maps can be converted to depth to provide a true scale structural interpretation at a specific level.
Seismic data is generally acquired and processed for the purpose of imaging seismic reflections for structural and stratigraphic interpretation. The quality of the seismic data that is ultimately used in the structural and stratigraphic interpretation depends on many different factors and varies from survey to survey. Steps that are omitted or not correctly completed in the data acquisition, data process and data interpretation stages can greatly affect the quality of the final images or numerical representation of the subsurface features. The quality of the seismic data directly affects the reliability of observations and numerical measurements made from the seismic data and affects the decisions that can or should be based on the seismic data.
Constructing accurate seismic images and corresponding earth models is important in making business or operational decisions relating to oil and gas exploration and reservoir management. For example, earth scientists use seismic images to determine where to place wells in subterranean regions containing hydrocarbon reservoirs. They also build models of the subsurface to create reservoir models suitable for reservoir fluid flow modeling. The quality of the business and operational decisions is highly dependent on the quality of the seismic images and earth models.
As described above, determining the quality of the seismic data used in seismic images and earth models is important. Prior art methodologies for determining seismic data quality generate only a single value for data quality for an entire seismic survey. Seismic data quality is not measured and the spatial variability of seismic quality within a particular survey is ignored. Prior art methodologies do not take into account that the quality of the seismic data may vary at differing points in a single seismic survey. Thus, one particular location in a seismic survey may have poor seismic data quality while another location in the same survey may have relatively good seismic data quality. The prior art does not differentiate between locations within a seismic survey based on seismic data quality. Thus, when seismic property estimates are combined with well data, a global correlation coefficient is used, and no account is given to the spatial variability of the quality of the seismic data.
Determining where the high or the low quality seismic data resides within a given seismic survey is important when decisions relating to oil and gas exploration and reservoir management are based in large part on seismic data.
There is a need for a method which determines the seismic data quality for a plurality of locations in a given seismic survey.